Effect of heavy metals on the carbon and nitrogen ratio in Avicennia marina from polluted and unpolluted regions

•Heavy metal accumulation in the mangroves altered their C/N ratio•Heavy metal enrichment in the root zone sediment affected the uptake of nitrogen by plants•Lower nitrogen uptake by plants resulted in higher C/N ratio

The accumulation of heavy metals and its impact on the C/N ratio of Avicennia marina of the Patalganga and Amba estuaries were studied. Vadinar was selected as a relatively uncontaminated location for comparison. Cd was accumulated in leaves of the Patalganga and Amba estuarine mangroves; however, at Vadinar it was accumulated and arrested only in roots and stems. Negative correlation of Cr, Cd, Pb and Zn with C in the mangroves suggested that their accumulation may lead to lowered C content in the mangroves. The average C/N ratios in mangroves of the inner Patalganga estuary, Patalganga mouth, and Amba estuary were found to be 80.1 ± 7.3, 105.8 ± 12.5 and 52.4 ± 3.4 respectively, whereas at Vadinar it was well within the expected range (26.4 ± 2.8). The results of the present study suggest: (i) metal accumulation, leading to less carbon content in plants; (ii) heavy metal enrichment in the root zone sediment, affecting the uptake of nitrogen by plants and resulting in altered C/N ratio.